1. Field of the Invention
The present invention relates to a printed circuit board and an optical transmission device.
2. Description of the Related Art
There is a demand to reduce an intensity of an unintentional electromagnetic wave generated from an optical transmission device to lower than a predetermined value. As the optical transmission device has many electromagnetic waves radiated from a built-in IC, development of a technology for effectively reducing the electromagnetic waves radiated from the IC becomes important.
To cope with this, JP 2970660 B proposes a technology for reducing the radiation of an unintentional electromagnetic wave due to a switching noise from an IC through arrangement of two chip capacitors with an appropriate interval between a power supply terminal of the IC and a power supply circuit of a printed circuit board.
However, the above-mentioned conventional technology may appropriately reduce the unintentional electromagnetic wave in a frequency range of from approximately 30 MHz to 1 GHz, but may not obtain a sufficient effect of reducing a high-frequency noise component, which is generated when handling a signal having a high frequency of, for example, equal to or higher than 20 GHz, which is used in a case of performing transmission at a high bit rate. Possible reasons therefor include the following two factors.
As a first factor, it is thought that, due to series inductance involved in the two chip capacitors, lowering of its impedance becomes hard to attain. As a second factor, even if the impedance of the series inductance involved in the chip capacitors may ideally be lowered, a loop current may generate a large amount of unintentional electromagnetic radiation. Accurate calculation of the electromagnetic radiation necessitates a simulation using a three-dimensional electromagnetic field analysis tool, but the description is made herein for the sake of simplicity by using an equation for a maximum electric field intensity in a far field (Mardiguian, Michel; translated by Kobayashi, Takehiko; “Controlling Radiated Emissions by Design”; Page 27; Equation (2.20)) based on a simple geometric model expressed by Equation (1) described below:E=(0.013·V·A·fMHz2)/D·Zc[V/m]  Equation (1)where fMHz represents a frequency expressed in MHz, A represents an area of a loop drawn by a closed current path, V represents a voltage (at the frequency fMHz) of an excitation source to be a noise, Zc represents a circuit impedance including an internal impedance of the excitation source, and D represents a distance from an observation position. As an example, it is assumed that: a current from the excitation source at the power supply terminal of the IC is 10 mA (that is, V/Zc=10 mA); a short circuit is established by an ideal decoupling capacitor in a position 4.0 mm apart from the power supply terminal of the IC; a printed circuit board has a thickness of 0.25 mm (that is, A=1.0 mm2) between a surface-layer wiring layer thereof and a ground wiring layer thereof; and there is no power supply wiring in an outside of the decoupling capacitor (that is, there is no propagation of a noise due to conduction through a power supply wiring and no noise radiation thereafter). When the maximum electric field intensity observed in the three-meter distant position in this case is calculated by Equation (1) described above, 33 dB(μV/m) is obtained in a case of the frequency of 1 GHz, while in a case where the frequency becomes as high as, for example, 20 GHz, the maximum electric field intensity is increased to 85 dB(μV/m), which is 400 times as high as 33 dB (that is, +52 dB). This indicates that even if the above-mentioned conventional technology producing effects in a frequency range of equal to or lower than 1 GHz is made to be ideal, the unintentional electromagnetic wave that is intense may be radiated with ease at the frequency of 20 GHz.